General Science | Page 7

for land and
sea breezes, and other daily phenomena.
The Gulf Stream illustrates the transference of heat by convection. A
large body of water is strongly heated at the equator, and then moves
away, carrying heat with it to distant regions, such as England and
Norway.
Owing to the shape of the earth and its position with respect to the sun,
different portions of the earth are unequally heated. In those portions
where the earth is greatly heated, the air likewise will be heated; there
will be a tendency for the air to rise, and for the cold air from
surrounding regions to rush in to fill its place. In this way winds are
produced. There are many circumstances which modify winds and
currents, and it is not always easy to explain their direction and velocity,
but one very definite cause is the unequal heating of the surface of the
earth.
13. Conduction. A poker used in stirring a fire becomes hot and heats
the hand grasping the poker, although only the opposite end of the
poker has actually been in the fire. Heat from the fire passed into the
poker, traveled along it, and warmed it. When heat flows in this way
from a warm part of a body to a colder part, the process is called
conduction. A flatiron is heated by conduction, the heat from the warm
stove passing into the cold flatiron and gradually heating it.
In convection, air and water circulate freely, carrying heat with them; in
conduction, heat flows from a warm region toward a cold region, but
there is no apparent motion of any kind.
Heat travels more readily through some substances than through others.
All metals conduct heat well; irons placed on the fire become heated
throughout and cannot be grasped with the bare hand; iron utensils are

frequently made with wooden handles, because wood is a poor
conductor and does not allow heat from the iron to pass through it to
the hand. For the same reason a burning match may be held without
discomfort until the flame almost reaches the hand.
Stoves and radiators are made of metal, because metals conduct heat
readily, and as fast as heat is generated within the stove by the burning
of fuel, or introduced into the radiator by the hot water, the heat is
conducted through the metal and escapes into the room.
Hot-water pipes and steam pipes are usually wrapped with a
non-conducting substance, or insulator, such as asbestos, in order that
the heat may not escape, but shall be retained within the pipes until it
reaches the radiators within the rooms.
The invention of the "Fireless Cooker" depended in part upon the
principle of non-conduction. Two vessels, one inside the other, are
separated by sawdust, asbestos, or other poor conducting material (Fig.
18). Foods are heated in the usual way to the boiling point or to a high
temperature, and are then placed in the inner vessel. The heat of the
food cannot escape through the non-conducting material which
surrounds it, and hence remains in the food and slowly cooks it.
[Illustration: FIG. 18.--A fireless cooker.]
A very interesting experiment for the testing of the efficacy of
non-conductors may be easily performed. Place hot water in a metal
vessel, and note by means of a thermometer the rapidity with which the
water cools; then place water of the same temperature in a second metal
vessel similar to the first, but surrounded by asbestos or other
non-conducting material, and note the slowness with which the
temperature falls.
Chemical Change, an Effect of Heat. This effect of heat has a vital
influence on our lives, because the changes which take place when food
is cooked are due to it. The doughy mass which goes into the oven,
comes out a light spongy loaf; the small indigestible rice grain comes
out the swollen, fluffy, digestible grain. Were it not for the chemical

changes brought about by heat, many of our present foods would be
useless to man. Hundreds of common materials like glass, rubber, iron,
aluminum, etc., are manufactured by processes which involve chemical
action caused by heat.


CHAPTER II
TEMPERATURE AND HEAT
14. Temperature not a Measure of the Amount of Heat Present. If two
similar basins containing unequal quantities of water are placed in the
sunshine on a summer day, the smaller quantity of water will become
quite warm in a short period of time, while the larger quantity will
become only lukewarm. Both vessels receive the same amount of heat
from the sun, but in one case the heat is utilized in heating to a high
temperature a small quantity of water, while in the second case the heat
is utilized in warming to a lower degree a larger quantity of water.